Theoretical study for ultrafast dynamics of photoexcited states in strongly correlated low-dimensional electron systems by analyzing nonlinear optical responses

自旋-电荷分离是否是强关联低维电子系统各种新颖物理性质产生的根本原因一直是人们讨论的热点，但至今仍存在争议。由于光仅耦合到电荷自由度，因此自旋动力学通过自旋和电荷自由度之间的耦合由光致电荷运动驱动。因此，超快光激发为研究自旋-电荷分离问题提供了一个理想的平台。考虑到这一点，我们研究了强关联低维电子系统中光激发态的动力学，得到了如下结果：瞬态四波混频强度敏感地反映了由于电荷和自旋运动引起的弛豫。这使我们能够阐明电荷和自旋自由度的相互作用。在二维的情况下，观察到一个良好分离的两步弛豫。这表明，在这种情况下，自旋和电荷自由度之间的耦合很弱。在一维情况下，自旋-电荷分离几乎严格成立。在实际的库仑相互作用参数下，我们发现了两个或两个以上的holons和doublons强束缚的光激发态，并且束缚的holons和doublons簇与周围的反铁磁自旋序共存. holon-doublon团簇态具有独特的电荷聚集机制，在反铁磁自旋背景下，来自自旋-自旋相互作用，以及在二维情况下，来自直接库仑相互作用。电荷聚集机制是合子和双光子奇异性质的产物。我们还发现，本质上不同的吸收光谱之间的一维和二维莫特绝缘体起源于自旋和电荷自由度之间的耦合的差异。

It has been heatedly discussed whether spin-charge separation is the root cause of various novel physical properties of strongly correlated low-dimensional electron systems, but it is still controversial. Since light couples only to the charge degrees of freedom, the spin dynamics is driven by the photoinduced charge motion through the coupling between spin and charge degrees of freedom. Therefore, the ultrafast optical excitation provides an ideal stage to investigate the problem of spin-charge separation. Considering these points, we have investigated the dynamics of photoexcited states in strongly correlated low-dimensional electron systems, and we have obtained the following results.We show that transient four wave mixing intensity sensitively reflects the relaxations due to the charge and spin motions. This enables us to clarify the interplay of charge and spin degrees of freedom. In the two-dimensional case, a well-separated two-step relaxation is observed. This shows that the coupling between the spin and charge degrees of freedom is weak in this case. In the one-dimensional case, spin-charge separation holds almost rigorously. We find the photoexcited states where two or more than two holons and doublons are strongly bound, and the cluster of bound holons and doublons coexists with the surrounding antiferromagnetic spin order, for the realistic Coulomb interaction parameters. The holon-doublon cluster states have a unique charge aggregation mechanism originating from spin-spin interaction in the antiferromagnetic spin background as well as from the direct Coulomb interaction in the two-dimensional case. The charge aggregation mechanism is the outcomes of exotic natures of holons and doublons. We also find that the essentially different absorption spectra between the one-dimensional and the two-dimensional Mott insulators originate from the difference in the coupling between spin and charge degrees of freedom.